1. Field of the Invention
This invention relates to a working method for an optical device wafer for dividing an optical device wafer, which has a plurality of regions partitioned by streets formed in a lattice on the surface of a substrate such as a sapphire substrate and has optical devices of a gallium nitride-based compound semiconductor or the like laminated in the partitioned regions, into the individual optical devices along the streets.
2. Description of the Related Art
An optical device wafer which has a plurality of regions partitioned by planned parting lines called streets formed in a lattice on the surface of a sapphire substrate and has optical devices of a gallium nitride-based compound semiconductor or the like laminated in the partitioned regions is divided into the individual optical devices such as light emitting diodes along the streets and is utilized widely in electric apparatus. The optical device wafer described above is ground and worked on the reverse face thereof by a grinding apparatus before it is divided along the streets so as to have a predetermined thickness. In recent years, it is demanded to make the thickness of optical devices less than 50 μm in order to implement reduction in weight and size of electric apparatus. However, if an optical device wafer is ground so as to be as thin as less than 50 μm, then this gives rise to a problem that a crack occurs therewith.
Meanwhile, cutting of an optical device wafer along streets is normally carried out by a cutting apparatus which includes a cutting blade which rotates at a high speed for cutting. However, since a sapphire substrate is a material which has high Mors hardness and is difficult to cut, it is necessary to lower the working speed, and therefore, there is a problem that the productivity is low. In recent years, as a method for dividing a wafer such as an optical device wafer along streets, it has been proposed to irradiate a pulsed laser beam of a wavelength having absorbency with respect to a wafer along a street to form a laser worked groove and apply external force along the laser worked groove to break the wafer along the street (for example, refer to Japanese Patent Laid-Open No. Hei 10-305420).
Meanwhile, as a method for dividing a wafer such as an optical device wafer along a street, also it has been proposed to use a pulsed laser beam of a wavelength having permeability with respect a wafer and irradiate the pulsed laser beam along a street with a focus point thereof adjusted to the inside of the wafer to form a altered layer continuously along the street in the inside of the wafer and then apply external force along the street whose strength is decreased by the formation of the altered layer to break the wafer along the street (for example, refer to Japanese Patent Laid-Open No. 2008-6492).
The optical device wafer described above is formed with a predetermined thickness by grinding the reverse face thereof before it is divided into individual optical devices. In recent years, it is demanded to set the thickness of an optical device to less than 50 μm in order to achieve reduction in weight and size of electric apparatus. However, there is a problem that, if the optical device wafer is ground so as to be thinner than 50 μm, then a crack occurs therewith.
Further, if a laser beam is irradiated along a street of an optical device wafer to form a laser worked groove or a altered layer and the optical device wafer is divided into individual optical devices along such streets along which such laser worked grooves or altered layers as described above altered, then since altered substance produced by the laser working remains on side faces (break faces) of the individually divided optical devices, there is a problem that the luminance of the optical devices drops and the die strength drops.